Dielectric layer composition

ABSTRACT

There is disclosed a paper which is suitable for use in electrostatic printing, said paper being coated on at least one side with a dielectric composition, which is useful particularly at relatively high humidities, consisting of 15 to 85 volume % of a copolymer of vinylidene chloride and at least one other olefinically unsaturated monomer, 8 to 77 volume % of an epoxy resin having an epoxy equivalent weight between 400 and 60,000, and, optionally, 0 to 42 volume % of a pigment.

United States Patent Camp et al. Dec. 2, 1975 [5 DIELECTRIC LAYER COMPOSITION 3.110.621 11/1963 Doggett ct al. 117/155 UA 3,l83,28l 5/1965 Clemens 260/837 R [75] lnvemors- Camly" -Q Lvermorer 3.236.683 2/1966 Berenbaum 6:61 260/836 Robert Lmdblom Walnut Creek- 3.385.730 5/l968 Relph 117/155 UA both of Calif.

[73] Assignee: The Dow Chemical Company, Primary E.t'am1'nerWilliam D. Martin Midland, Mich. Assistant Examiner-Janyce A. Bell [22] Filed June 29 1973 Attorney, Agent. or Firm.l. Roger Lochhead; Gary D. Street {21] App]. No.: 374,847 4 Related US. Application Data 5 ABSTRACT 1 2 -32 Ser No 173299 Aug 197] Pat No There 1s dlsclosed a paper whlch 1s su1table for use 1n electrostatic printing, said paper being coated on at [52] Cl. 428/413; 427/121. 428/514 least one side with a dielectric composition. which is 511 Int. Cl .f. 33121 27/38 useful Particularly relatively high humidifies- [58] Field of Search 260/41 R. 83-6 837- sisting of 3 volume of a copmymer of vinyl 117/223, 155 UA, 155 R, 61 U2 161 dene chloride and at least one other olefinically unsat- 161/184 25 urated monomer, 8 to 77 volume 71 of an epoxy resin having an epoxy equivalent weight between 400 and [56] References cued 60,000. and. optionally. 0 to 42 volume of a pig- UNITED STATES PATENTS 3,008,914 ll/l96l Fry 260/837 CA Claim, N0 Drawings DIELECTRIC LAYER COMPOSITION This is a divisional application of application Ser. No. 173,299, filed Aug. 19, 1971, now US. 3,779,982.

BACKGROUND OF THE INVENTION Electrographic paper for use in electrostatic printing,

said paper being formulated to look like ordinary pal0 per, consists of a paper substrate generally made conductive by the addition of electroconductive resins or salts, on the top of which is placed a highly resistive dielectric resin coating. Generally these papers contain a pigment, the purpose of which is to reduce the gloss of the polymer coating and to give the paper a flat, mattetype finish. The resulting paper can be of any size, depending upon the requirements of the machine.

In a typical printing process, the paper is passed through an electrographic printer. Voltage in the range of 500-900 volts.is applied across the dielectric coating. One process has an electrode making contact with the electroconductive substrate, and the other electrode, consisting of styli which are selectively activated by programmed impulses, is in proximity with, but not touching, the dielectric coating. An electrical discharge occurs across the air gap, resulting in an electrostatic image on the surface of the paper.

An alternative process charges the dielectric layer through one stylus, with the return path through another electrode on the same side of the paper.

The dielectric layer functions as a charged capacitor, the charged paper then being passed through a toner containing oppositely charged particles. The particles adhere to the electrostatic image, resulting in a visible print.

There are many dielectric materials which, when coated on paper, will accept a static charge and produce a toned image at about 50% room humidity. Any fairly good dielectric will hold sufficient charge long enough to be toned out in a few minutes to produce'a visible print.

But when the coated papers are run through high speed printers (5800 lines per minute) and when the humidity to which the paper is subjected begins to vary over a range of to 85 percent, each component of the electrographic system begins to have critical requirements.

At low humidity the conductivity of the base sheet must be provided solely by the electroconductive resin with which it is impregnated. Conductivity of the base sheet determines the time required to transfer charge to the dielectric. The dielectric coating must be capable of being charged in a matter of about 50 microseconds. As .the humidity goes up to 80 percent and higher, other problems become paramount. For instance, the papers begin to exhibit curl problems and the charge leaks off and through the paper much more quickly. In addition, parts of the dielectric coating lose their dielectric strength and break down at voltage levels which are inadequate for printing.

Past experience has shown that many soft resins, such as vinylidene chloride copolymers, while they do not curl, will not receive and maintain an electrical charge under high humidity conditions. On the other hand, while many hard resins, such as epoxy resins, will receive and maintain a charge, they exhibit excessive curl under these conditions.

SUMMARY OF THE INVENTION Dielectric coatings have now been found which have desirable properties when said coatings and their associated substrate are exposed to relatively high humidities, i.e., above about 50%.

The coatings of this invention exhibit desirable dielectric properties under said conditions, i.e., they exhibit l good charging characteristics, (2) good charge retention characteristics, and (3) the ability to withstand changes in the dimensions of the substrate.

The coatings of this invention consist essentially of 15 to 85 volume preferably 32 to 58%, ofa copolymer of vinylidene chloride and at least one other olefinically unsaturated comonomer polymerizable therewith; 8 to 77 volume 70, preferably 24 to 50%, of an epoxy resin having an epoxy equivalent weight of about 400 to 60,000; and, optionally, 0 to 42 volume preferably 10 to 25%, of a pigment.

The coating may be placed on one or both sides of an electrographic paper by known techniques, i.e., impregnating, padding, dipping, spraying, coating, or the like, in an amount effective as a dielectric layer, resulting in papers suitable for use in electrostatic printing processes. For instance, a coat weight of about 1 to 15 pounds per ream (3,000 ft. preferably about 3 to 6 pounds per ream, .is generally suitable. The approximate optimum volume ratio of components for use herein at high relative humidities, i.e., above about 50%, is about 49%copolymer, about 37% epoxy resin and about 14% pigment.

DETAILED DESCRIPTION OF THE INVENTION Copolymers suitable for the practice of the present invention include copolymers of vinylidene chloride containing at least about weight percent, preferably about 80%, but less than about 96% vinylidene chloride and at least one other olefinically unsaturated monomer copolymerisable therewith.

Among the myriad of suitable ethylenic comonomers are substituted and unsubstituted styrenes of the formula i=CH wherein each R and R is, independently, I-I, Cl, Br, F or a straight or branched-chain alkyl group of up to 8 carbon atoms (such as methyl, propyl, butyl or octyl), and n is 0 to 5. Examples include 0-, mand p-methyl styrenes; alpha-methyl styrene; 2,3-,2,4- and 2,5- dimethyl styrenes; alpha-chlorostyrene; alpha-ethyl styrene; o-, mand p-bromo-, chloroor fluorostyrenes; isopropenyl toluene; 2,3-, 2,4- and 2,5-dichloro-, dibromo-and difluorostyrenes; and the like.

Also suitable are vinyl naphthalene; methacrylonitrile; acrylonitrile; 2-chloroacrylonitrile; vinylchloride; acrylate and alkylacrylate esters, such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, Z-ethyI-hexylacrylate, dodecyl acrylate, 2-chloroethyl acrylate, Z-hydroxy-ethyl acrylate, Z-hydroxy-propyl acrylate, 2-chloropropyl acrylate, 2,2-dichloro-isopropyl acrylate, phenyl acrylate, cyclohexyl acrylate, methyl alpha-chloroacrylate, methyl methacrylate, ethyl methacrylate, 2-sulfo-ethyl methacrylate and methyl ethacrylate; vinylidene cyanide; vinyl esters, such as vinyl acetate, vinyl chloroacetate, vinyl propio- (iii nate, vinyl butyrate, vinyl laurate and vinyl stearate; CaSO silicas (such as amorphous. diatomaceous. vinyl ethers, such as vinyl methyl ether and vinyl isobumagnesium silicate talc, aluminum silicate china clay, tyl ether; vinyl ketones. such as vinyl'methyl ketone, mica. bentonite, wollastonite and lorite), lithopone, vinyl hexyl ketone and methyl isopropenyl ketone; iso- Zinc sulfide. antimony oxide. barium sulfate. calcium butylene; l-butene; vinylidene halides. such as vinyli- 5 carbonate. ground limestone. toluidine red toner, dene chlorofluoride; N-vinyl compounds. such as N- hansa yellow. chrome yellow. phthalocyanine green or vinyl pyrrole. N-vinyl carbazole. N-vinyl indole and N- blue, molybdate orange, dinitraline orange toner. carvinyl succinimide; acrolein. methacrolein; acrylamide: bon black. and the like. Other pigments which could be methacrylamide; N-methylol acrylamide; allyl comsubstituted for the above will be obvious to those in the pounds. such as allyl alcohol, methallyl alcohol. allyl l0 coating arts. Lithopone is the preferred pigment for use acetate. allyl methacrylate, allyl lactate. allyl alphain the dielectric coating when a pigment is used. hydroxyisobutyrate. and allyl acrylate; and the like. The above coatings may suitably be prepared by dis- Copolymers which are suitable herein haveamolecusolving the copolymer and epoxy resin, in any selar weight which is described by a-relative viscosity of quence. in a suitable solvent in the desired ratio. A pigbetween about 1.2 and 3.5, preferably 1.6 to 3.0, for a ment, if desired, may be dispersed into the solution by 1% tetrahydrofuran solution at C as determined by known techniques. such as sand milling. Ambient con- ASTM Test ,Dl243-60. ditions are generally suitable "for the above process.

Preferred copolymers include those which are 75 to Suitable solvents include ketones, such as methyl 95% vinylidene chloride and 5 to 257r vinyl chloride or ethyl ketone, acetone, cyclohexanone and isophorone;

acrylonitrile. and tetrahydrofuran, dimethylformamide, ethyl ace- Suitable epoxy resins are those which have an epoxy tate, and the like.

equivalent weight of about 400 to 60.000 and are genocmlHCHp wherein each Z is a chemical bond, O. S. E l

-S S*' To 160 ml. of methyl ethyl ketone in a wide-mouth bottle equipped with a magnetic stirrer was added 40 0 f gm. of a copolymer of 80% vinylidene chloride and u I 20% of acrylonitrile, said copolymer having relative a viscosity of 2.57 in a 1% solution of tetrahydrofuran at 25C. The solution was stirred for one hour, and then r 10 gm. of an epoxy resin of the above formula'wherein CH2 or n is 0, Z is isopropylidene and the epoxy equivalent 40 weight is about 1800, was added. the stirring continued for another hour. 40 gm. of lithopone pigment was then added, and sand-milled (20-30 mesh milling sand) at 3700 RPM for about 15 minutes. The sand was separated by filtering the coating/milling sand mixture through a conicalpaper paint filter. Q i H or The above mixture was then applied to a conductivebase paper of 133 megohms/ I] by the use of a wire- 0 wound rod, typically a No. 18 rod. Howeventhe rod size and solids content of the mixture were both varied to control the holdout from the paper and the coating weight. each X is Cl or Br; Films were used with coating weights of from 3 lbs. to each n is 0 to 2; and m is about 1 to 210. l4 lbs/ream. Commercial films are typically 5 These resins are generally formed by the reaction, in lb./ream. but lab samples are usually less perfect due to the presence ofa suitable catalyst. of a bisphenol and a batch-wise coating, and somewhat heavier films were diglycidyl ether of a bisphenol or an epihalohydrin in a necessary to achieve comparable dielectric strength ratio such that a product with an epoxy equivalent and charge retention. weight of about 400 to 60,000 is formed. this mode of The samples were evaluated by charging the dielecpreparation being well known to the art. tric film with 600 volts against anetched halftone mag- Preferred epoxy resins are those wherein Z is isopronesium test plate. A high impedance 600 voltpower pylidene. Q is 2.3-epoxypropyl and the epoxy equivasupply had the negative lead connected to the test plate lent weight is between about 800 and 6,000. and the positive lead connected to a piece of foil. The Pigments known to the art to be useful when incorpopaper was interposed between the plate and the foil,

rated into dielectric coatings are suitable herein. As an the dielectric side facing the plate. Intimate contact example. the following may be used: white leads (such was achieved by the use of a rubber roller.

as basic carbonates. sulfates and silicates). zinc oxides The electrostatic image thus achieved was converted (such as acicular. nodular and 35% leaded). titanium into an optical image by the use of a commercially dioxides (such as rutile. anatasev and 30% TiO /7c available liquid toner.

Charge acceptance and retention was measured by a Most Associates Stati-Tester. To measure charge retention, the charged paper was placed in the machine where it was given a charge by corona discharge. Sub- 6 Test prints were also visually inspected, compared and rated Excellent (E), Good (G), Fair (F) or Poor (P) according to their uniformity of toning.

Each test was carried out on a print which had been sequent to the charging period, the machine measured 5 immediately toned (0 min.) and toned minutes after the surface charge (Vo) with an electrometer. The macharging (30 min. and at 50 and 80% relative humidichine then measured surface charge 2 minutes later ties. However, regarding the results below. it is to be (V). The paper was thusly tested under ambient humidunderstood that the comparisons must be made bearing ity conditions (-7c) and at 85% relative humidity. in mind the differences in coat weights. Lithopone was The table below summarizes this data for various 10 used as pigment, the amounts being shown in the table.

Print Quality RH. 80% R.H. Density Unifomity Density L'niformity Vol. 7t Vol. 71 Vol. "/1 Coat 0 30 30 0 30 0 30 Polymer Epoxy Pigment Wcight Min Min Min Min Min Min Min Min 1. 73 0 27 7.7 0.8 0.4 E E 0.5 0.2 P P 2. 51 25 25 8.8 0.9 0.4 E E 0.8 0.2 G F 3. 43 32 25 8.9 1.2 0.7 E E 0.6 0.5 G F 4. 32 44 24 (1.7 1.2 0.7 E E 0.7 0.4 F P 5. 22 23 7.5 1.0 0.7 c; o 0.7 0.2 P P e. 0 79 21 7.5 0.7 0.6 P F 0.2 0.1 P P coat weights and different ratios of polymer and epoxy. It is apparent from the above that, particularly at high The pigment used was lithopone, the amount used (i.e., humidities, the blends of polymer and epoxy being shown in the table. The coat weights given are of this invention are superior to either alone. approximate, it being understood that variations in coat 25 We claim: weight will result in variations in results. I. A paper suitable for use in electrostatic printing TABLE I.

5 lbs/Ream" 8 lbs/Ream" Ex. v01. 71 W1. /1 v01. /1 35% RH. 8571 R.H. 35% RH. 35% RH. No. Polymer Epoxy Pigment V,, V,,/V V,, \/',,/V V.,/V '..l\'

"coat-weight of dielectric film Regarding Table l, the most desired ratio of V,,/V is processes comprising an electrographic paper coated 1, showing perfect charge retention. However, it is on at least one side with an effective amount of a diclear that various blends of polymers and epoxy resins 40 electric coating composition consisting essentially of are superior in both initial charging and charge reten- A. 15 to 85 volume 7: of a copolymer of 70 to 96 tion over either alone, it being understood that optiweight vinylidene chloride, the remainder being mum ratios of the two may easily be determined for at least one other olefmically unsaturated comonovarious humidity conditions and coat weights. mer polymerizable therewith, said copolymer hav- The ability of a dielectric coating to accept and retain 45 ing a molecular weight described by a relative visa charge ultimately is judged by print contrast on a toned sheet. The contrasts of the dielectric composition of the instant invention was measured by a microscope, which allowed small areas 1 .4 mm to be observed. A light meter was used to measure the reflected cosity of between about 1.2 and 3.5 for a 1% tetrahydrofuran solution at 25C.;

B. 8 to 77 volume 7: of an epoxy resin having an epoxy equivalent weight of about 400 to 60,000, said epoxy resin being of the formula O x n x n X x n CH CHCH O Z OCH iHCH O Z 0Q. 

1. A PAPER SUITABLE FOR USE IN ELECTROSTATIC PRINTING PROCESSES COMPRISING AN ELECTROGRAPHIC PAPER COATED ON AT LEAST ONE SIDE WITH AN EFFECTIVE AMOUNT OF A DIELECTRIC COATING POSITION CONSISTING ESSENTIALLY OF A. 15 TO 85 VOLUME % OF A COPOLYMER OF 70 TO 96 WEIGHT % VINYLIDENE CHLORIDE, THE REMAINDER BEING AT LEAST ONE OTHER OLEFINICALLY UNSATURATED COMONOMER POLYMERIZABLE THEREWITH, SAID COMPLYMER HAVING A MOLECULAR WEIGHT DESCRIBED BY A RELATIVE VISCOSITY OF BETWEEN ABOUT 1.2 AND 3.5 FOR A 1% TETRAHYDROFURAN SOLUTION AT 25*C.; B. 8 TO 77 VOLUME % OF AN EPOXY RESIN HAVING AN EPOXY EQUIVALENT WEIGHT OF ABOUT 400 TO 60,000, SAID EPOXY RESIN BEING OF THE FORMULA 